The invention concerns a method for correction of a phase error in an MR image. The invention can, for example, be used to calculate the phase error that arises during the imaging due to system instabilities or system imprecisions. Furthermore, the invention can be used for splitting of signals from fatty tissues from signals from aqueous tissues.
Imaging by way of nuclear magnetic resonance, i.e., magnetic resonance tomography (“MR tomography”) finds an ever wider field of application in diagnostics. In addition to the innocuousness of the radiated fields and in addition to the fact that the image plane can be freely selected in MR tomography, an MR signal has the advantage that the MR signal can be represented by complex numbers and not just a scalar quantity as is, for example, the case given conventional x-ray images or in computer tomography.
In an MR image, the magnitude of the MR signal and the phase (i.e., the direction of the magnetization vector that corresponds to the signal) exist for each image point. Given standard magnitude imaging, the phase information is not used. However, there are also applications for which the phase information is of greater importance. For example, the phase information is used in order to acquire information about the blood flow. The phase information can, for example, also be used to depict the vessel structure or for temperature imaging.
Given use of the phase information, in many applications, the fundamental problem exists that not only does the desired phase curve exist resulting from the radiated radio-frequency pulses (RF pulses), but also includes other phase effects due to field inhomogeneities or a temporal change of the external magnetic field BO.
However, for many applications it is desirable to be able to quantify the system-dependent phase errors in order to be able to subsequently remove them when using the phase information in the MR image.
One possible application field of phase-corrected MR images lies, for example, in the splitting of the signals from two different tissue components when the examined tissue comprises two different components (e.g., fat and water). Given two different tissues with a different chemical-related shift, a different magnetic field results at the nucleus which leads to different resonance frequencies. In the signal acquisition, this leads to different phase angles of the two components. The most prominent representatives of two different tissue types in the magnetic resonance signal are fat and water, although other applications are also possible. The resonance frequencies of fat and water differ by approximately 3.5 ppm (parts per million). In many clinical MR applications, it is desirable to suppress the signal of the fat since the fat signal is typically very strong and, for example, can occlude lesions.
A further application field of phase-corrected MR images exists given pulse sequences with a previously inserted 180° inversion pulse, i.e., what are known as inversion recovery acquisitions, or given other acquisitions in which a phase change of 180° can exist.
U.S. Patent Publication No. 2005/0165296 A1 describes a phase correction method that is based on what is known as the region growing algorithm, with which the system-dependent phase error is calculated in that phase gradients within the MR image are calculated and this phase gradient is used for a correction algorithm in order to remove system-dependent phase errors. However, two further phase images must be calculated for this correction algorithm: a phase gradient image in the one direction of the image plane as well as a phase gradient in the second direction of the image plane, which makes the calculation complicated.